In many industries x-ray inspection is being used routinely for inspection of products in the manufacturing environment. The simplest form of x-ray imaging is transmission imaging. In x-ray transmission imaging, illumination is projected through an object and the imaging signal results from a subtractive process, i.e.; what is imaged is the far field of the illumination minus any of the illumination that was absorbed, reflected or scattered. Such images can be quickly generated with relatively inexpensive equipment. However, detail needed to detect a flaw in a part can be obscured in such images when the defective feature is masked by an area of highly absorbing material that overlaps the defective feature in the image. For example, the solder joints on printed circuit boards are often examined via x-ray inspection to detect defective solder joints. Unfortunately, a large solder joint on one surface of the board can interfere with the image of a smaller joint on the other side of the board if the larger joint shadows the smaller joint.
The shadowing problem can be substantially reduced when three-dimensional x-ray images are formed of the object. In essence, the object is divided into thin slices that are individually examined. Hence, one object that obscures another object in one slice will not obscure the other object in another slice. Such three-dimensional images are often used in medical diagnostic work.
Unfortunately, the high cost of this equipment and its relatively low throughput has inhibited the use of three-dimensional scanners for such high volume applications as parts inspection. Three-dimensional scanners such as CT scanners require that a large number of views of the object be taken from a large number of different angles. This data must then be combined to provide the three-dimensional image in which the part is modeled as a three dimensional array of volume elements called voxels. The three-dimensional image must then be analyzed to detect flaws in the part.
In a typical CT scanner, the x-ray source and the detector are fixed relative to one another and are rotated around the object. The time needed to generate a three-dimensional image with such a system makes such systems impractical in high-speed part scanning applications. In addition, the cost of the hardware and the floor space required further discourage the use of such systems in industrial inspection applications. Finally, the computational costs of the analysis of the three-dimensional images also increases the cost of such systems.
Broadly, it is the object of the present invention to provide an improved x-ray inspection apparatus and method.
This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.